1. Field of the Invention
The present invention relates to a spring steel with improved resistance to hydrogen embrittlement.
2. Description of the Related Art
The chemical compositions of spring steels are specified in JIS G3565 to 3567, G4801 and the like. By use of these spring steels, various springs are manufactured by the steps of: (1) hot-rolling each spring steel into a hot-rolled wire rod or bar (hereinafter, referred to as “rolled material”); and drawing the rolled material to a specified diameter and then cold forming the wire into a spring after oil-tempering; or (2) drawing the rolled material or peeling and straightening the rolled material, heating and forming the wire into a spring, and quenching and tempering it.
Recently, there have been strong demands toward the enhancement of the stress of a spring as a part of measures of achieving small and light springs in order to reduce exhaust gas or fuel consumption. For example, there is required a high strength spring steel of which strength after quenching and tempering is HRC52 or greater. However, as the strength of a spring is enhanced, the sensitivity against defects is generally increased. Particularly, since the high strength spring used in a corrosion environment is deteriorated in corrosion fatigue life, there is a possibility of causing an early breakage. It is being thought that the reason why corrosion fatigue life is reduced is that corrosion pits on the surface of a spring act as stress concentration sources which accelerate the generation and propagation of fatigue cracks. To prevent the reduction of corrosion fatigue life, corrosion resistance must be improved by the addition of elements such as Si, Cr and Ni. However, these elements are also effective to enhance quenching and tempering, and when used in large amounts they produce a supercooling structure (martensite, bainite, etc.) in the rolled material. This requires a softening heat treatment such as annealing before drawing the rolled material. Therefore, the number of processing steps is increased, leading to an increase in the manufacturing cost.
Recently, a technology for improving both corrosion fatigue characteristics and workability has been developed (U.S. Pat. No. 5,776,267). This proposes refining and dispersing fine precipitates of carbide, nitride, sulfides such as Ti, Zr, Ta, Hf and the like in a spring steel. This is so because the finely dispersed precipitates can trap diffusive hydrogen in the spring steel and suppress hydrogen from diffusing and carrying prior austenite grains, consequently preventing hydrogen embrittlement. According to this disclosure, when the prior austenite grain is 20 μm or smaller, the carbide, nitride, sulfide precipitating in a crystal grain boundary become extremely fine as well. This hardly exerts an adverse effect on toughness or fatigue property of the spring steel, but enhances diffusive hydrogen trapping.
Besides the above-described U.S. Pat. No. 5,776,267, other techniques for improving resistance to hydrogen embrittlement (Japanese Patent Publication Nos. 3429164 and 3219686 and Japanese Patent Laid-Open No. 2005-23404, etc.). Japanese Patent Publication No. 3429164 disclosed a method for improving resistance to hydrogen embrittlement by securing an amount of Ti carbo-nitride production by replacing S with CuS, knowing that the existence of S reduces an amount of Ti carbo-nitride production effective for hydrogen supplementation. Meanwhile, Japanese Patent Publication No. 3219686 disclosed a method for improving resistance to hydrogen embrittlement by reducing the formation of MnS based inclusions. It also teaches that the resistance to hydrogen embrittlement can be enhanced even more by reducing size and volume ratio provided that the same amount of MnS based inclusions was used. Lastly, according to Japanese Patent Laid-Open Publication No. 2005-23404, by suitably balancing the contents of Cr, Ti, and V, hydrogen infiltration into a spring steel can be prevented and thus, corrosion fatigue resistance of the spring steel is remarkably improved.